Transient high voltages can induce harmful currents and voltages in electronic circuits and electric equipment. Normally, these transient voltages are caused by electrostatic discharge, lightning or inductive power surges.
Overvoltage protection materials and devices have been developed which aim to protect such electronic and electrical equipment from these transient high voltages. The overvoltage materials have non-linear electrical resistance characteristics, specifically having a very high electrical resistance (e.g. >30 MOhm) at a normal operating voltages (e.g. <200 V for normal electronic instrument), but switching to an essentially conducting state when subject to a transient high voltage. The voltage at which the material changes from its high resistance state (or “off state”) to its conducting state (or “on state”) is known as the threshold value, or trigger voltage. It is necessary for the materials to have a very fast switching time between the high resistance state and conducting state in order to provide adequate protection for the electronic or electrical equipment being protected. An acceptable switching time is usually of the order of nanoseconds, and is preferably less than one nanosecond. The device returns back to its normal high resistance state after the transient high voltage threat has passed.
The term overvoltage protection material usually refers to a composite material containing a conductive material within a polymer matrix system. The overvoltage protection material may comprise only a two components (the conductive material and the polymer matrix), or can contain other components such as a semi-conductive material or a non-conductive material.
Conventional methods for preparing overvoltage protection materials include solvent processing or polymer condensation processes. Usually, the finished polymeric systems should be a network structure or a thermoset in order to achieve good thermal properties. Therefore, both of these processing techniques should be carried out in the presence of a cross-linking component.
(a) Solvent Processing Method
One method of making thermoset plastics suitable for use as overvoltage protection materials is a solvent processing method, as described in U.S. Pat. Nos. 4,977,357 and 5,248,517. The overvoltage conductive materials are prepared by mixing a dissolved silicone polymer solution, nickel powder, silicon carbide powder; inorganic filler and cross-linking agent (e.g. organic peroxide) to form a paste like composition. This material is then applied to a prepared substrate which contains metal electrodes, to form an uncured device. The device is then cured in an oven at 125° C. for 4 hours.
However, this solvent processing technique has a number of disadvantages. First, the polymer must be dissolved in a suitable solvent, the choice of which is critical to the success of the reaction. Not all solvents can be used (e.g. all chlorinate and some ketone-type solvents are unsuitable), and it can take a great deal of time to determine which solvents are suitable. Second, a relatively large amount of solvent must be removed (e.g. sometime as much as 1:1, solid to solvent weight content) under the high temperature curing step. Removal of solvent can be damaging to the device, generating voids within polymer matrix and causing a resulting degradation in performance of the final overvoltage protection device. Third, the processing temperature condition required to evaporate the solvent is relatively high, and the curing time is relatively long (e.g. approximately 4 hours at 125° C.).
(b) Polycondensation Process Method
U.S. Pat. No. 5,928,567 discloses a liquid conductive material designed to protect electrical components from high pulse static electricity. The process comprises combining a solvent-free liquid silicone polymer composition (General Electric RTV11), a conductive metallic powder, a non-conductive inorganic powder and a catalytic amount of the curing agent (e.g. dibutyl tin dilaurate) and mixing these components in a conventional multi-blade mixer. The paste like material is applied to an appropriate substrate, and is then cured in a convection oven at 80° C. for 2 hours. The process is described as solvent free because no conventional organic solvent is added in the process described.
However, while no solvent is added to the mixture, the process itself will generate solvent as a by-product. In the process specifically described in this patent, the polymerization process is a common condensation reaction between di- or tri-hydroxy polydimethylsiloxane and di- or tri-methoxy polydimethylsiloxane prepolymers. A general reaction scheme is shown below in Scheme 1.

From this it can be seen that, although a solvent is not required as an initial component of the reaction mixture, some solvent (in this case methanol) will be generated during the course of the curing reaction. In this instance, one mole of solvent is generated for every mole of product. During the curing reaction the solvent present in the mixture will evaporate and in doing so will generate unwanted voids or bubbles in the polymer matrix. This in turn diminishes the ability of the device to respond properly during conditions of transient high voltage.
From a consideration of the prior art processes it can be seen that performance of overvoltage protection devices is reduced due to the presence of solvent either as an added component of the reaction mixture, or as a by-product formed during the curing process. It is therefore an object of the invention to provide an improved and convenient method for preparing overvoltage protection materials. In particular, it is an object of the invention to provide a solvent-free, or “clean”, process for preparing an overvoltage protection material suitable for protecting electronic circuits and electrical devices from transient high voltages, where no solvent is added to the reaction mixture, and where no substantially solvent is generated during the course of the process. It is another object of the invention to provide an improved overvoltage protection material. It is also an object of the invention to provide new overvoltage protection devices comprising this improved overvoltage protection material.